Flexible molded polyurethane foam requires mechanical crushing to open foam cells and prevent shrinkage and to improve dimensional stability of the foam pad. Current mechanical methods for cell opening consist mainly of crushing, vacuum rupture or time pressure release.
Upon demold, mechanically crushing and breaking the polyurethane foam cells enables the polyurethane foam to be more dimensionally stable. Another method of breaking the cells is vacuum crushing which involves drawing a vacuum on the finished polyurethane foam causing cell rupture. The overall effect of these methods is reduced foam shrinkage.
Other mechanical attempts have been made to achieve dimensionally stable foam, such as decreasing cycle production times. For example, demolding the polyurethane foam in three minutes as compared to four minutes will dramatically improve the dimensional stability. Another method for producing dimensionally stable foam is time pressure release (TPR). TPR comprises opening the mold during the curing process to release the internal pressure and then reclosing for the duration of the cure time. The sudden release of the internally generated pressure bursts the cell windows, thereby obtaining an open cell foam.
Mechanical methods usually result in incomplete or inconsistent cell opening and require a flexible molded foam producer to invest in additional machinery. A chemical method for cell opening would be preferred.
The current chemical methods all have drawbacks such as requiring high levels often as high as 1-5 weight parts per hundred parts polyol (pphpp) or adversely affecting the foam physical properties.
U.S. Pat. No. 3,454,504 discloses a cell opening agent for producing polyurethane foam which is a liquid polypropylene or a polybutene.
U.S. Pat. No. 4,431,455 discloses an organic polyisocyanate composition containing a liquid organic polyisocyanate and a mixture of a wax and a liquid ester. The composition, which is preferably applied in the form of an aqueous emulsion, is used for the manufacture of sheets or molded bodies, such as chipboard, fiberboard and plywood, by the hot-pressing of a lignocellulosic material and promotes release of the material from the press.
U.S. Pat. No. 4,751,253 discloses a cell opening, dimensionally stabilizing agent for making flexible foam which comprises an ester reaction product of a long chain acid with polyethylene or polypropylene glycols and/or contains free acid to provide for a desired acid value.
U.S. Pat. No. 4,929,646 discloses flexible polyurethane foams prepared using certain high molecular weight, high functionality poly(oxyethylene) compounds as cell openers and softeners. In Example 1 Cell Opener A, a nominally 6.9 functional random copolymer of 75% ethylene oxide and 25% propylene oxide having an approximate molecular weight of 35,000, was added to the polyurethane formulation as a 70/30 mixture with water.
U.S. Pat. No. 5,614,566 discloses rigid foams which are opened celled prepared by reacting the components in the presence of liquid, higher molecular weight, unsaturated hydrocarbons, which are free of groups capable of reacting with isocyanates, such as polybutadiene and polyoctenylene.
WO 96/37533 discloses preparing rigid polyurethane foams using an emulsified polyol mixture comprising (a) a polyol formulation comprising a polyol having an OH value of from 150 to 500, (b) a blowing agent, (c) a cell opening agent which is a divalent metal salt of a long chain fatty acid having a softening point from about 100-180.degree. C., and (d) an acid, the mixture having droplets of the cell opening agent having an average mean diameter of less than about 50.mu. stably suspended in the polyol mixture.
U.S. Pat. No. 4,936,917 discloses a method for making a polyurethane foam using a water-based mold release composition comprising an aqueous dispersion of at least one release effective substance and a poly(siloxane-glycol) surfactant.
In a technical sales book distributed by Dow Plastics to customers (1991 ed by Ron Herrington and Kathy Hock, page 2.31), it is stated that "Known additives for inducing cell opening include silicone based antifoamers, waxes, finely divided solids and certain polyether polyols made using high concentrations of ethylene oxide." No further discussion is given in this publication regarding the types of polyurethane applications these are useful for or types of waxes that are necessary to achieve cell opening. In particular, a table is provided (page 3.19) of known cell opening additives and there is no listing of a wax-like compound. Also, this reference does not disclose a method of introducing the wax into the foam composition.